When oils (e.g. cooking oil, frying oil, fat, shortening, etc.) are exposed to high temperatures, particularly in the presence of oxygen and/or water, oxidative reactions can take place that result in degradation of the oils. Thus, oil quality is often monitored in restaurant kitchens, so as to determine whether the oil is still suitable for use.
A parameter often used to evaluate oil quality is the free fatty acid content of the oil. Mlinar and Neumayer, for example, disclose in U.S. Pat. No. 4,654,309 an article for testing liquid for free fatty acid content. An organic liquid to be tested is contacted with the article and any color change in the article after the passage of sufficient time is observed.